This recipe was actually shared with me from Tiff at Wright Writes!
Cook dry pasta (Macaroni=2 cups dry; Egg Noodles=4-5 cups dry; etc) until al dente. Do not overcook pasta!
While pasta is cooking, make cheese sauce:
4 Tbs Margarine or butter
4 Tbs Flour
1/2 tsp Salt
1/4 tsp pepper
1/2 tsp Dry Mustard
2 cups milk
1-1 1/2 cups shredded cheddar cheese
In microwavable bowl, melt butter, about 15-30 seconds uncovered. Stir in flour, salt, pepper, mustard, and milk with a whisk. Microwave uncovered 2-3 minutes, stirring every minute with whisk or fork until thickened. You may need to cook it longer to get it to the right thickness you want. When it is thick enough, add 1 cup of cheese gradually and stir until all cheese is melted. I usually microwave it another 30-60 seconds just to make sure it's all melted well.
Drain pasta. Spray casserole dish with cooking spray and pour pasta into dish. Pour cheese sauce over top of pasta and just kinda mix it together a little to coat the pasta a little. (you don't have to stir it but I like to just make sure everything is nice and coated) then sprinkle remaining cheese on top and bake at 350-400 degrees until cheese is melted and kinda bubbly. Serve!